Background on using site-mixed concrete in construction
Until around 15 years ago, mixing concrete on site (also known as site-mixed concrete) was a common practice in most construction projects. It was used to cast columns, load-bearing walls, and even slabs without distinction. At the time, this approach was considered normal and acceptable, and the culture of laboratory testing and strict supervision was not as widespread as it is today. With the advancement of engineering knowledge and the increased reliance on codes and standards, it became clear that this approach poses a real risk to the quality and safety of structures, especially when used in primary structural elements.
The real difference between site-mixed concrete and ready-mix concrete
The core issue with site-mixed concrete is not the type of cement or aggregate used; rather, it lies in the lack of precise control over mix proportions—especially the water-to-cement ratio. In ready-mix plants, mixes are controlled using accurate measuring systems and are subject to continuous supervision and periodic testing to ensure consistent quality and compliance with the design specifications. On site, however, mixing is often left to workers’ judgment and ease of execution, making concrete quality inconsistent and even variable from one batch to another within the same project.
A practical experiment to prove the difference in compressive strength
To prove this in a practical way away from theory, we carried out a real on-site experiment. A site-mixed concrete batch was prepared, and test cubes were taken from it using the same method followed with ready-mix concrete. They were then prepared and transported to an accredited laboratory to conduct compressive strength tests. The aim of this experiment was to compare the actual results of the site mix with the results we are used to obtaining from ready-mix concrete in previous projects, under the same conditions and according to the code.
The mix used and the impact of water on the results
The adopted mix ratio was 3:2:1, meaning 1 cement to 2 coarse aggregate to 3 sand—a well-known mix that, in theory, should deliver a compressive strength between 20 and 25 MPa. However, the decisive factor that negatively affects this strength is the water ratio, as workers typically add extra amounts to make mixing, pouring, and finishing easier. Despite our attempt to reduce water as much as possible and adhere to standards, controlling water on site remains limited compared to what is available in ready-mix concrete plants.
Laboratory test results and comparison with ready-mix concrete
The laboratory test results showed a clear and shocking difference. While with ready-mix concrete we would reach a compressive strength of around 22–25 MPa after 7 days of casting, the compressive strength of the site-mixed concrete did not exceed 8 MPa on the seventh day. The results indicate that even after 28 days, this sample would barely reach 10 MPa, which is extremely low compared to the minimum required for structural works.
Engineering assessment of mixing concrete on site
These results conclusively confirm that site-mixed concrete cannot be relied upon in load-bearing structural elements and is, from an engineering standpoint, unacceptable. Low compressive strength means the element cannot safely تحمل the design loads, placing the entire structure within the risk zone—especially over time as it is exposed to variable loads and different environmental factors.
Why did old buildings withstand despite weak concrete?
Many people ask a logical question: how are old buildings still standing today even though they were built with low-strength concrete? The answer lies in the structural design philosophy, which relies on
using high safety factors that assume the worst possible scenarios. Structural elements were designed based on strengths far higher than the expected actual loads.
For example, if a slab is required to be designed to carry a live load of 100 tons, calculations may be performed on the basis of 120 tons or more. These factors are not meant to permanently compensate for poor execution, but to provide a limited safety margin. When a building is executed with weak concrete, this margin is consumed from day one, leaving no room to accommodate changes in use or any unforeseen future conditions.
The impact of poor execution on the service life of buildings
Over time, this depletion of safety factors leads to permanent stress in structural elements, so cracks and deflection begin to appear and may develop into serious structural problems. That is why we see today buildings that are no more than 40–50 years old already needing evacuation or strengthening, while buildings constructed according to proper standards are still standing after much longer periods.
The importance of ready-mix concrete in modern construction
This highlights the importance of relying on ready-mix concrete in modern projects, as it ensures consistent quality and precise compliance with design specifications, and reduces risks resulting from human error on site. It also directly contributes to extending the building’s service life, achieving the highest levels of safety and sustainability, and is a fundamental pillar of any sound and sustainable construction.
Conclusion
Proper construction does not start with appearance nor end with finishing; it starts with the quality of the concrete itself, because it is the element that carries everything above it. Using ready-mix concrete is no longer an optional extra, but an engineering necessity that cannot be compromised in any project that seeks safety and quality in the long term.
